Semiconductor device and method for fabricating the same

ABSTRACT

A method for fabricating semiconductor device includes the steps of first forming a gate structure on a substrate, forming a spacer adjacent to the gate structure, forming a recess adjacent to the spacer, trimming part of the spacer, and then forming an epitaxial layer in the recess. Preferably, the semiconductor device includes a first protrusion adjacent to one side of the epitaxial layer and a second protrusion adjacent to another side of the epitaxial layer, the first protrusion includes a V-shape under the spacer and an angle included by the V-shape is greater than 30 degrees and less than 90 degrees.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to a method for fabricating semiconductor device, and more particularly to a method of using etching process to trim spacers before forming epitaxial layer.

2. Description of the Prior Art

In order to increase the carrier mobility of semiconductor structure, it has been widely used to apply tensile stress or compressive stress to a gate channel. For instance, if a compressive stress were to be applied, it has been common in the conventional art to use selective epitaxial growth (SEG) technique to form epitaxial structure such as silicon germanium (SiGe) epitaxial layer in a silicon substrate. As the lattice constant of the SiGe epitaxial layer is greater than the lattice constant of the silicon substrate thereby producing stress to the channel region of PMOS transistor, the carrier mobility is increased in the channel region and speed of MOS transistor is improved accordingly. Conversely, silicon carbide (SiC) epitaxial layer could be formed in silicon substrate to produce tensile stress for gate channel of NMOS transistor.

Current approach of forming MOS transistor having epitaxial layer typically conducts a lightly doped ion implantation process to form lightly doped drains (LDDs) in the substrate adjacent to two sides of the spacer before forming epitaxial layers. However, lightly doped drains formed by ion implantation process is unable to accurately control the dopant distribution within the lightly doped drains thereby resulting in leakage and short channel effect (SCE). Hence, how to improve the current fabrication to resolve this issue has become an important task in this field.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a method for fabricating semiconductor device includes the steps of first forming a gate structure on a substrate, forming a spacer adjacent to the gate structure, forming a recess adjacent to the spacer, trimming part of the spacer, and then forming an epitaxial layer in the recess. Preferably, the semiconductor device includes a first protrusion adjacent to one side of the epitaxial layer and a second protrusion adjacent to another side of the epitaxial layer, the first protrusion includes a V-shape under the spacer and an angle included by the V-shape is greater than 30 degrees and less than 90 degrees.

According to another aspect of the present invention, a semiconductor device includes a gate structure on a substrate, a spacer adjacent to the gate structure, and an epitaxial layer adjacent to the spacer. Preferably, the epitaxial layer comprises a protrusion having an angle greater than 30 degrees under the spacer.

According to yet another aspect of the present invention, a semiconductor device includes a gate structure on a substrate, a spacer adjacent to the gate structure, a first epitaxial layer adjacent to the spacer, a second epitaxial layer having a V-shape on the first epitaxial layer, and a third epitaxial layer on the second epitaxial layer.

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-6 illustrate a method for fabricating a semiconductor device according to an embodiment of the present invention.

FIG. 7 illustrates a structural view of a semiconductor device according to an embodiment of the present invention.

DETAILED DESCRIPTION

Referring to FIGS. 1-6, FIGS. 1-6 illustrate a method for fabricating a semiconductor device according to an embodiment of the present invention. As shown in FIG. 1, a substrate 12 is provided, and gate structures 14 and 16 are formed on the substrate 12. In this embodiment, the formation of the gate structures 14 and 16 could be accomplished by sequentially forming a gate dielectric layer, a gate material layer, and a hard mask on the substrate 12, conducting a pattern transfer process by using a patterned resist (not shown) as mask to remove part of the hard mask, part of the gate material layer, and part of the gate dielectric layer through single or multiple etching processes, and stripping the patterned resist. This forms gate structures 14 and 16 on the substrate 12, in which each of the gate structures 14 and 16 includes a patterned gate dielectric layer 18, patterned gate material layer 20, and patterned hard mask 22. It should be noted that to emphasize the formation of epitaxial layer between the two gate structures 14 and 16 in the later process, two transistors are presented in this embodiment and only part of the transistor elements including the region between two gate structures 14 and 16 are shown in the following figures.

In this embodiment, the substrate 12 could be a semiconductor substrate such as a silicon substrate, an epitaxial substrate, a silicon carbide (SiC) substrate, or a silicon-on-insulator (SOI) substrate, but not limited thereto. The gate dielectric layer 18 could include silicon oxide (SiO₂), silicon nitride (SiN), or high-k dielectric material; the gate material layer 20 could include metal, polysilicon, or silicide; the material of hard mask 22 could be selected from the group consisting of SiO₂, SiN, SiC, and SiON.

According to an embodiment of the present invention, a plurality of doped wells or shallow trench isolations (STIs) could be selectively formed in the substrate 12. Despite the present invention pertains to a planar MOS transistor, it would also be desirable to apply the process of the present invention to non-planar transistors, such as FinFET devices, and in such instance, the substrate 12 shown in FIG. 1 would be a fin-shaped structure formed atop a substrate 12.

Next, at least one spacer is formed on sidewalls of each of the gate structures 14 and 16, and an ion implantation process such as a tilted angle implantation process could be conducted to implant dopants into the substrate 12 adjacent to two sides of the gate structures 14, 16 for forming pocket regions 26. In this embodiment, the spacer formed on sidewalls of each of the gate structures 14, 16 is preferably a composite spacer further including a spacer 28 disposed or directly contacting sidewalls of the gate structures 14, 16 or gate electrodes and a spacer 30 disposed on sidewalls of the spacer 28, in which each of the inner spacer 28 and the outer spacer 30 includes an I-shape cross-section. In this embodiment, the inner spacer 28 and the outer spacer 30 could be made of same material or different materials as both the spacers 28, 30 could include silicon oxide (SiO₂), silicon nitride (SiN), silicon oxynitride (SiON), silicon carbon nitride (SiCN), or combination thereof. Preferably, the pocket regions 26 and the type of transistor device being fabricated include different conductive types. For instance, if a PMOS transistor were fabricated in this embodiment, the pocket regions 26 preferably include n-type dopants, but not limited thereto.

Next, as shown in FIG. 2, a first etching process is conducted to form initial recesses 32 in the substrate 12 adjacent to two sides of the spacer 30. In this embodiment, the first etching process preferably includes dry etching process and the first etching process could further includes three stage of etching processes, in which the first stage etching process includes a vertical direction etching process conducted to remove part of the substrate 12, the second stage etching process includes a horizontal direction etching process conducted to remove part of the substrate 12, and the third stage etching process includes another vertical direction etching process conducted to remove part of the substrate 12 for forming the recesses 32.

Specifically, the first stage etching process preferably includes hydrogen bromide (HBr) and/or helium (He), in which the flow of HBr and/or He is approximately 200/20 standard cubic centimeter per minute (sccm) and the duration of the process is approximately 11 seconds. The second stage etching process preferably includes chlorine gas (Cl₂) and/or ammonia (NH₃), in which the flow of Cl₂ and NH₃ is approximately 50/10 sccm while the duration of the process is approximately 15 seconds. The third stage etching process preferably includes hydrogen bromide (HBr) and/or helium (He), in which the flow of HBr and/or He is approximately 200/20 sccm and the duration of the process is approximately 6-10 seconds.

Next, as shown in FIG. 3, a second etching process is conducted to trim the spacer 30 for reducing the overall thickness or width of the spacer 30. In this embodiment, the second etching process preferably includes another dry etching process and gases used in the second etching process could include trifluoromethane (CHF₃), tetrafluoromethane (CF₄), or combination thereof, in which the flow of CHF₃ or CF₄ is approximately 35/60 sccm and duration of the etching process is about 0.05 ns. It should be noted that the etching gas used during the second etching process not only trims the spacer 30 but also removes part of the substrate 12 adjacent to two sides of the spacer 30 to form voids 34 or indentations directly under the spacer 30. Despite the width of the spacer 30 is slightly reduced during the second etching process, the bottom surface of the spacer 30 is still even with the surface of the substrate 12 so that the top surface of the voids 34 is also even with the surface of the substrate 12 directly under the gate structures 14, 16 or the bottom surface of the spacer 28. According to other embodiment of the present invention, the voids 34 could not only expose the bottom surface of the spacer 30 but could also be extended inward to expose the bottom surface of the spacer 28, which is also within the scope of the present invention.

Next, as shown in FIG. 4, a third etching process is conducted to isotropically expand or enlarge the initial recesses 32 for forming recesses 36. In this embodiment, the third etching process preferably includes wet etching process, in which the wet etching process could be accomplished using etchant including but not limited to for example ammonium hydroxide (NH₄OH) or tetramethylammonium hydroxide (TMAH). It should be noted that the formation of the recesses 36 is not limited to wet etching process disclosed in this embodiment. Instead, the recesses 36 could also be formed by single or multiple dry etching and/or wet etching processes, which are all within the scope of the present invention. According to an embodiment of the present invention, each of the recesses 36 could have various cross-section shapes, including but not limited to for example a circle, a hexagon, or an octagon. Despite the cross-section of each of the recesses 36 in this embodiment pertains to be a hexagon, it would also be desirable to form the recesses 36 with aforementioned shapes, which are all within the scope of the present invention.

Next, as shown in FIG. 5, a selective epitaxial growth (SEG) process is conducted to form buffer layers 38 and epitaxial layers 40 in the recesses 36 while filling the voids 34 completely. In this embodiment, the combination of buffer layers 38 and epitaxial layers 40 preferably constitute a hexagon shaped cross-section and a top surface of the epitaxial layers 40 is slightly higher than a top surface of the substrate 12. Taking an epitaxial layer adjacent to one side of the gate structure such as the epitaxial layer 40 between the gate structures 14, 16 as an example, two protrusions including a first protrusion 42 and a second protrusion 44 are formed adjacent to two sides of the epitaxial layer 40 by filling the voids 34 with epitaxial layer 40 as the two protrusions 42, 44 contact the bottom surfaces of the spacers 30 directly. Preferably, each of the first protrusion 42 and the second protrusion 44 includes a V-shape directly under the spacer 30, the included angle φ of the V-shape is preferably greater than 30 degrees and less than 90 degrees, and the depth D of each of the first protrusion 42 and the second protrusions 44 is less than ⅕ of the entire depth of the epitaxial layer 40 including but not limited to for example 5 nm to 30 nm.

In this embodiment, the epitaxial layers 40 could also be formed to include different material depending on the type of transistor being fabricated. For instance, if the MOS transistor being fabricated were to be a PMOS transistor, the epitaxial layers 40 could be made of material including but not limited to for example SiGe, SiGeB, or SiGeSn. If the MOS transistor being fabricated were to be a NMOS transistor, the epitaxial layers 40 could be made of material including but not limited to for example SiC, SiCP, or SiP. Moreover, the SEG process could also be adjusted to form a single-layered epitaxial structure or multi-layered epitaxial structure, in which heteroatom such as germanium atom or carbon atom of the structure could be formed to have gradient while the surface of the epitaxial layers 40 is preferred to have less or no germanium atom at all to facilitate the formation of silicide afterwards. Since the present embodiment pertains to the fabrication of PMOS transistor, the germanium content within the epitaxial layers 40 is preferably between 30% to 50% while the concentration of boron in the epitaxial layers 40 is preferably between 1.0×10²° atoms/cm³ to 1.0×10²¹ atoms/cm³.

It should be noted that in contrast to using ion implantation approach for forming lightly doped drains (LDDs) in current process, the present invention preferably omits the process of conducting ion implantation process for forming LDDs but instead employs an in-situ doping approach to form doped regions with even concentration distribution during the formation of the epitaxial layers 40. Preferably, the doped regions formed in the first protrusion 42 and the second protrusions 44 are serving as lightly doped drains 24. After the LDDs 24 are formed, an ion implantation process could be conducted to implant dopants into substantially central region of the epitaxial layers 40 such as regions not directly under the spacer 30 and outside the first protrusion 42 and the second protrusion 44 for forming source/drain regions 46, in which the concentration of the source/drain regions 46 is greater than the concentration of the lightly doped drains 24 formed in the first protrusion 42 and second protrusion 44 while the two regions 24, 46 share dopants of same conductive type. Next, a cap layer 48 is formed on the epitaxial layers 40, in which the cap layer 48 made of pure silicon is preferably grown upward along the sidewalls of the spacer 30 and a top surface of the cap layer 48 preferably includes a planar surface.

Next, as shown in FIG. 6, a contact etch stop layer (CESL) (not shown) and an interlayer dielectric (ILD) layer 54 are formed on the gate structure 14, 16, and a planarizing process such as chemical mechanical polishing (CMP) process is conducted to remove part of the ILD layer 54 and part of the CESL to expose hard masks 22 so that the top surfaces of the hard masks 22 and ILD layer 54 are coplanar.

Next, a replacement metal gate (RMG) process is conducted to transform the gate structures 14, 16 into metal gates. For instance, the RMG process could be accomplished by first performing a selective dry etching or wet etching process, such as using etchants including but not limited to for example ammonium hydroxide (NH₄OH) or tetramethylammonium hydroxide (TMAH) to remove the hard masks 22, gate material layer 20 and even gate dielectric layer 18 for forming recesses (not shown) in the ILD layer 54. Next, a selective interfacial layer 56 or gate dielectric layer, a high-k dielectric layer 58, a work function metal layer 60, and a low resistance metal layer 62 are formed in the recesses, and a planarizing process such as CMP is conducted to remove part of low resistance metal layer 62, part of work function metal layer 60, and part of high-k dielectric layer 58 to form gate structures 14, 16 made from metal gates 64, 66. In this embodiment, each of the gate structures 14, 16 or metal gates fabricated through high-k last process of a gate last process preferably includes an interfacial layer 56 or gate dielectric layer (not shown), a U-shaped high-k dielectric layer 58, a U-shaped work function metal layer 60, and a low resistance metal layer 62.

In this embodiment, the high-k dielectric layer 58 is preferably selected from dielectric materials having dielectric constant (k value) larger than 4. For instance, the high-k dielectric layer 58 may be selected from hafnium oxide (HfO₂), hafnium silicon oxide (HfSiO₄), hafnium silicon oxynitride (HfSiON), aluminum oxide (Al₂O₃), lanthanum oxide (La₂O₃), tantalum oxide (Ta₂O₅), yttrium oxide (Y₂O₃), zirconium oxide (ZrO₂), strontium titanate oxide (SrTiO₃), zirconium silicon oxide (ZrSiO₄), hafnium zirconium oxide (HfZrO₄), strontium bismuth tantalate (SrBi₂Ta₂O₉, SBT), lead zirconate titanate (PbZr_(x)Ti_(1-x)O₃, PZT), barium strontium titanate (Ba_(x)Sr_(1-x)TiO₃, BST) or a combination thereof.

In this embodiment, the work function metal layer 60 is formed for tuning the work function of the metal gate in accordance with the conductivity of the device. For an NMOS transistor, the work function metal layer 60 having a work function ranging between 3.9 eV and 4.3 eV may include titanium aluminide (TiAl), zirconium aluminide (ZrAl), tungsten aluminide (WAl), tantalum aluminide (TaAl), hafnium aluminide (HfAl), or titanium aluminum carbide (TiAlC), but it is not limited thereto. For a PMOS transistor, the work function metal layer 60 having a work function ranging between 4.8 eV and 5.2 eV may include titanium nitride (TiN), tantalum nitride (TaN), tantalum carbide (TaC), but it is not limited thereto. An optional barrier layer (not shown) could be formed between the work function metal layer 60 and the low resistance metal layer 62, in which the material of the barrier layer may include titanium (Ti), titanium nitride (TiN), tantalum (Ta) or tantalum nitride (TaN). Furthermore, the material of the low-resistance metal layer 62 may include copper (Cu), aluminum (Al), titanium aluminum (TiAl), cobalt tungsten phosphide (CoWP) or any combination thereof.

Next, part of the high-k dielectric layer 58, part of the work function metal layer 60, and part of the low resistance metal layer 62 are removed to form recesses (not shown), hard masks 68 are then formed into the recesses, and a planarizing process is conducted so that the top surfaces of the hard masks 68 and ILD layer 54 are coplanar. The hard masks 68 could be made of material including but not limited to for example SiO₂, SiN, SiON, SiCN, or combination thereof.

Next, a contact plug formation could be conducted to form contact plugs 70 electrically connected to the source/drain regions 46. In this embodiment, the formation of contact plugs 70 could be accomplished by removing part of the ILD layer 54 and part of the CESL to form contact holes (not shown), and then depositing a barrier layer (not shown) and a metal layer (not shown) into the contact holes. A planarizing process, such as CMP is then conducted to remove part of the metal layer, part of the barrier layer, and even part of the ILD layer 54 to form contact plugs 70, in which the top surface of the contact plugs 70 is even with the top surface of the ILD layer 54. In this embodiment, the barrier layer is selected from the group consisting of Ti, Ta, TiN, TaN, and WN, and the metal layer is selected from the group consisting of Al, Ti, Ta, W, Nb, Mo, and Cu.

Referring again to FIG. 6, FIG. 6 further illustrates a structural view of a semiconductor device according to an embodiment of the present invention. As shown in FIG. 6, the semiconductor device preferably includes at least a gate structure 14 made of metal gate 64 disposed on the substrate 12, spacers 28, 30 disposed adjacent to the gate structure 14, pocket regions 26 disposed in the substrate 12 adjacent to two sides of the gate structure 14, and epitaxial layers 40 disposed in the substrate 12 adjacent to two sides of the spacers 30, in which each of the epitaxial layers 40 includes two protrusions and each of the protrusions includes an angle greater than 30 degrees directly under the spacer 30.

Specifically, the epitaxial layer adjacent to one side of the gate structure such as the epitaxial layer 40 between the gate structures 14, 16 includes a first protrusion 42 adjacent to one side of the epitaxial layer 40 and a second protrusion 44 adjacent to another side of the epitaxial layer 40, in which the first protrusion 42 is disposed in the substrate 12 adjacent to one side of the gate structure 14 from another perspective while the second protrusion 44 is disposed in the substrate 12 adjacent to one side of the gate structure 16. Viewing from a more detailed perspective, the first protrusion 42 directly contacts the bottommost surface of the spacer 30 adjacent to the gate structure 14, the second protrusion 44 directly contacts the bottommost surface of the spacer 30 adjacent to the gate structure 16, each of the first protrusion 42 and the second protrusion 44 includes a V-shape directly under the spacer 30, the included angle φ of the V-shape is preferably greater than 30 degrees and less than 90 degrees, and the depth D of each of the first protrusion 42 and the second protrusions 44 is less than ⅕ of the entire depth of the epitaxial layer 40 including but not limited to for example 5 nm to 30 nm.

Referring to FIG. 7, FIG. 7 illustrates a structural view of a semiconductor device according to an embodiment of the present invention. As shown in FIG. 7, it would be desirable to first form first epitaxial layers 72 in the recesses 32 filling or without filling the voids 34 after forming the buffer layer 38 shown in FIG. 5, conduct a fourth etching process by using hydrofluoric acid (HCl) to remove part of the first epitaxial layers 72, forms second epitaxial layers 74 having V-shape cross-section on the first epitaxial layers 72, forms third epitaxial layers 76 on the second epitaxial layers 74, and then forms the cap layer 48 on the third epitaxial layers 76.

It should be noted in order to improve current leakage of the device, it would be desirable to form the first epitaxial layers 72 with in-situ dopants for forming source/drain regions 46 in the first epitaxial layers 72 and the third epitaxial layers 76 and form the second epitaxial layers 74 having another in-situ dopants with opposite conductive type in the second epitaxial layers 74 for forming lightly doped drains 24. In this embodiment for fabricating PMOS transistor, the source/drain regions 46 in the first epitaxial layers 72 and the third epitaxial layers 76 preferably include p-type dopants while the lightly doped drains 24 in the second epitaxial layers 74 include n-type dopants.

Overall, the second epitaxial layers 74 includes a substantially V-shape cross-section and similar to the epitaxial layers 40 from the aforementioned embodiment, two protrusions including a first protrusion 42 and a second protrusion 44 serving as lightly doped drains 24 are formed adjacent to two sides of the second epitaxial layer 74 by filling the voids 34 with second epitaxial layers 74 as the two protrusions 42, 44 contact the bottom surfaces of the spacers 30 directly. Preferably, each of the first protrusion 42 and the second protrusion 44 includes a V-shape directly under the spacer 30, the included angle φ of the V-shape is preferably greater than 30 degrees and less than 90 degrees, and the depth D of each of the first protrusion 42 and the second protrusions 44 is less than ⅕ of the overall depth from the first epitaxial layers 72 to the third epitaxial layers 76 including but not limited to for example 5 nm to 30 nm. In this embodiment, the germanium content within the first epitaxial layers 72 and/or the third epitaxial layers 76 is preferably between 30% to 50% while the concentration of boron in the epitaxial layers 72 and 76 is preferably between 1.0×10²° atoms/cm³ to 1.0×10²′ atoms/cm³. The germanium content within the second epitaxial layers 74 is preferably between 30% to 50% while the concentration of n-type dopants such as phosphorus in the epitaxial layers 74 is preferably between 1.0×10¹⁶ atoms/cm³ to 1.0×10²¹ atoms/cm³.

Typically, an extra lightly doped ion implantation process is conducted after using ion implantation process to form pocket regions 26 and before using the aforementioned first etching process to form recesses 32 in the substrate to form lightly doped drains in the substrate adjacent to two sides of the spacer. Since lightly doped drains formed by ion implantation process is unable to accurately control the concentration distribution of dopants within the lightly doped drains thereby resulting in leakage and short channel effect (SCE), the present invention preferably omits the process of conducting ion implantation process for forming LDDs but instead employs an in-situ doping approach to form lightly doped regions (such as the first protrusion 42 and second protrusion 44) with uniform concentration distribution during the formation of the epitaxial layers (such as the second epitaxial layer 74). Moreover, another embodiment of the present invention involves performing an etching process to trim or thin the outer spacer 30 so that the lightly doped drains form by in-situ doping and epitaxial layers could be formed closer to the gate structures thereby improving the performance of the device.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims. 

1. A method for fabricating a semiconductor device, comprising: forming a gate structure on a substrate; forming a spacer adjacent to the gate structure; performing a first etching process to remove the substrate for forming a first recess adjacent to the spacer; performing a second etching process to trim the spacer for forming a void directly under the spacer adjacent to the first recess without expanding a width of the first recess; and forming a first epitaxial layer in the first recess.
 2. The method of claim 1, further comprising: performing a third etching process to expand the first recess for forming a second recess; and forming the first epitaxial layer in the second recess.
 3. The method of claim 2, wherein the first etching process comprises a dry etching process.
 4. The method of claim 2, wherein the second etching process comprises a dry etching process.
 5. The method of claim 2, wherein the third etching process comprises a wet etching process.
 6. The method of claim 2, further comprising: performing a fourth etching process to remove the first epitaxial layer; forming a second epitaxial layer with in-situ dopants on the first epitaxial layer; and forming a third epitaxial layer on the second epitaxial layer.
 7. The method of claim 6, wherein the first epitaxial layer and second epitaxial layer comprise dopants of opposite conductive type.
 8. The method of claim 6, wherein the second epitaxial layer comprises a first V-shape.
 9. The method of claim 1, further comprising: a first protrusion adjacent to one side of the first epitaxial layer; and a second protrusion adjacent to another side of the first epitaxial layer.
 10. The method of claim 9, wherein the first protrusion comprises a second V-shape under the spacer.
 11. The method of claim 10, wherein an angle included by the second V-shape is greater than 30 degrees and less than 90 degrees.
 12. A semiconductor device, comprising: a gate structure on a substrate; a spacer adjacent to the gate structure; an epitaxial layer adjacent to the spacer, wherein the epitaxial layer comprises a hexagon; and a protrusion having an angle greater than 30 degrees under the spacer and adjacent to the hexagon.
 13. The semiconductor device of claim 12, further comprising: a first protrusion adjacent to one side of the epitaxial layer; and a second protrusion adjacent to another side of the epitaxial layer.
 14. The semiconductor device of claim 13, wherein the first protrusion comprises a V-shape under the spacer.
 15. The semiconductor device of claim 14, wherein an angle included by the V-shape is greater than 30 degrees and less than 90 degrees.
 16. The semiconductor device of claim 14, wherein a depth of the first protrusion is less than ⅕ the thickness of the epitaxial layer.
 17. A semiconductor device, comprising: a gate structure on a substrate; a spacer adjacent to the gate structure; a first epitaxial layer adjacent to the spacer; a second epitaxial layer on the first epitaxial layer and contacting the spacer directly, wherein a bottom surface of the second epitaxial layer comprises a first V-shape and a top surface of the second epitaxial layer comprises a second V-shape; and a third epitaxial layer on the second epitaxial layer.
 18. The semiconductor device of claim 17, wherein the first epitaxial layer and second epitaxial layer comprise dopants of opposite conductive type.
 19. The semiconductor device of claim 17, wherein the second epitaxial layer and third epitaxial layer comprise dopants of opposite conductive type.
 20. The semiconductor device of claim 17, wherein the first V-shape is directly under the spacer. 